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Pine Trees Help Reconstruct a Long-Ago Drought

By Justin Gillis November 7, 2011 1:26 pmNovember 7, 2011 1:26 pm

Daniel Griffin/Laboratory of Tree-Ring ResearchA cross section of a bristlecone pine with its annual growth rings.

As they struggle to understand modern climate change, scientists have long realized the importance of reconstructing past climate variability. That’s the only way to gain a sense of perspective, to understand how anomalous modern climate events are or aren’t.

Yet the field of “paleoclimatology” has been plagued by controversy, with reconstructions of past temperatures emerging as a particular target for climate-change contrarians.

The essential problem, of course, is that the temperature record that tells us the Earth is warming goes back only to about 1850, when observation stations systematically began keeping such data, and most other climate records are even shorter and spottier. Reconstructing climates prior to 1850 requires the use of “proxies” for temperature and rainfall, like tree rings and pollen grains in lake sediments.

Mark Losleben/
Laboratory of Tree-Ring ResearchCody Routson, a researcher, taking a sample from a bristlecone pine.

Now comes a new installment in this literature that may be sobering for many people, particularly in the American Southwest. Led by Cody C. Routson, a graduate student at the University of Arizona, the paper supplies strong evidence of a half-century “megadrought” in the Southwest that occurred in the second century, with the driest stretch being a 25-year period from A.D. 148 to 173. (A summary of the paper is here, and readers with American Geophysical Union credentials can get the full paper here. A news release about the paper is here.)
Mr. Routson and his co-authors, Connie A. Woodhouse and Jonathan T. Overpeck, focused on bristlecone pines, the longest-lived of all trees. They found a stand near Summitville, Colo., covering more than 2,000 years, and were able to obtain specimens of dead wood and thin cores from living trees. (The trees were not harmed by the research.)

Analyzing tree rings and matching them with previous work, they found what they consider to be compelling evidence a long dry spell in the Southwest stretching through the first four centuries of the Christian era and punctuated by more acute dry stretches.

Scientists already knew the Southwest was prone to long, severe megadroughts far worse than anything that has happened there since European settlement of the region began. Some of the same scientists involved in this work helped document such a drought in the medieval era. And some previous research had already pointed toward the likelihood of a second-century megadrought. But with the fresh evidence, the Arizona scientists believe the case is nailed.

“Our record brings these previous studies together to show that is event was indeed real and widespread,” Mr. Routson told me by e-mail.

Evidence of the causes is of course quite thin, but the scientists speculate that one factor may have been a broad pattern of warmth in the Northern Hemisphere, possibly caused by an uptick in the sun’s energy output. This warm spell is often called the “Roman Warm Period,” and while it has not been definitively proven to exist, evidence for the idea is growing.

Climate-change contrarians love to cite the Roman Warm Period and its counterpart, the Medieval Warm Period, as proof that there is nothing unusual about today’s rising temperatures. But mainstream climate scientists say these earlier episodes, while intriguing, do not necessarily shed light on the question of whether modern-day human activity is causing the climate to warm, as their evidence suggests it is.

“The evidence for human-induced warming is overwhelming, but not the focus of this study,” Mr. Routson said.

Scientists expect temperatures in the coming century to rise substantially compared with these earlier warm periods. Since the earlier periods were associated with severe droughts and climate disruptions in some parts of the world, they warn of a potential replay — except that the coming megadroughts could be worse.

For people in the Southwest, living through a severe drought and its consequences right now, that cannot be good news. “Showing that the most extreme droughts in the Southwest have predominantly occurred during warmer intervals in our past raises important questions about what we should expect in an even warmer future,” Mr. Routson said.

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